1. Field of the Invention
This invention relates to the preparation of aerial survey photomaps and more particularly to method and apparatus for making in-scale photographic maps.
2. Description of the Prior Art
Aerial photographs of geographic terrain are commonly used as maps and many applications are more useful than the known planimetric maps to record ground information, which must otherwise be obtained by conducting costly ground surveys and costly office drafting. The many details that are shown on an aerial photograph that may be converted to an in-scale photomap are obtained at a significantly less cost than that of conducting a ground survey. For these reasons, it is beneficial to use in-scale aerial photographs (photomaps) in the recording of utility lines and transmission lines.
In-scale aerial photomaps are commonly used by the U.S. Army Corp. of Engineers in the study of navigable rivers and particularly, photomaps of locks and dams and shore areas. Sanitary and water supply engineers utilize in-scale aerial photographic maps for systems studies, construction plans in land development studies and the recording of rights-of-way. Many commercial businesses utilize aerial photographs for recording data relevant to property holdings and property improvements.
An aerial photograph has the general appearance of a map, but the conditions that must exist to make a photograph an in-scale accurate photomap do not prevail. The only points that will be in their proper relative positions on an aerial photograph are the points on the datum plane, which is the horizontal plane selected as a reference level for computing the scale of the photograph. Objects that are higher than the datum plane and are not vertically beneath the camera will be located too far from the center of the resulting photograph and as a result, distortions of objects on the ground appear on the photograph. On the other hand, objects that are lower than the datum plane and are not vertically beneath the camera will be located too near the center of the resulting photograph. This characteristic of aerial photographs is called relief displacement or displacement of images because of relief.
If an object on the ground has appreciable height, an aerial photograph will show the image leaning away from the principal point of the photograph, unless the object happens to be vertically beneath the lens of the camera. Similarly, if a long, straight, structure such as a fence, extends over a hill its image on a aerial photograph will be bowed away from the principal point. If such a structure extends across a valley, then its image will be bowed toward the principal point. Other irregularities are caused by lens abberations, lens disturbances and appear in the form of barrel shaped distortion, a pin cushion shaped distortion, and a tangential shaped distortion. The airplane taking the photographs is rarely if ever exactly level. This further distorts the photograph into an oblong rather than circular shape.
Various known methods and apparatus have been proposed for preparing aerial photographic maps that eliminate distortion of images on that photograph due to relief displacement and other distortions.
U.S. Pat. No. 3,692,407 discloses a method and apparatus for making aerial survey and other photographs to scale. This is accomplished by first preparing a photographic reproduction by photographing an area and projecting the photograph against a reflecting surface. The image is reflected from the surface to an image surface. The reflecting surface is selectively deformed to eliminate distortions in the image of the photograph projected onto the image surface. The reflecting surface is a stretched, flexible and therefore, can be moved into a non-planar configuration. Control devices are individually manipulated to disturb the surface of the stretched reflecting mirror to change the position of the location of key points which are projected onto the image surface. The image surface includes an easel that holds photographic materials in overlying relationship. The easel supports a flat, resilient cushion which receives a rigid transparent member overlying the cushion. A sheet of glass holds one or more flat sheets in position on the easel.
Initially, a template of an area being mapped is prepared. The template includes principal or key points which are fixed by a field survey. An aerial photograph of the area corresponding to the area on the template is projected onto the template positioned on the easel. The reflective surface is selectively deformed to move the reflected image of the key point into register with the corresponding key point on the template. Thereafter, a photographic material is placed in the same plane as the template and is exposed to produce a photographic representation of the area which is suitably scaled.
Preparation of in-scale aerial photographs prepared by the above described method and apparatus have been found useful in mapping linear strip maps of pipe lines, sanitary sewers, water lines and the like. While the known device has been effective in producing in-scale aerial photographs of relatively confined linear strip areas, it is not operable for mapping of large geographical areas such as the mapping of statewide public utility service areas, cities and countries. By mounting a stretched flexible mirror for projecting the photographic image onto photographic material supported by an easel, the mirror is tension mounted. The mirror is sufficiently flexible to be warped/deformed by using controls at the edges that engage the mirror and, for example, move it forward to distort or warp the engaged area of the mirror.
The mirror is initially stretched and mounted in tension. However, when the controls are manipulated to selectively deform an area of a stretched mirror, additional tension is induced in the mirror. This results in a rippling action which propagates throughout the mirror. Thus other parts of the mirror interact with the area of the mirror being deformed. While one area of the stretched mirror is deformed to move a key feature of the reflected image into register with the template on the easel another reflected image is moved out of register with the template. Then efforts to counter distort a reflective image create more disturbances in other effected areas of the reflected image. With no tension reduction or outlet available in the stretched mirror efforts to correct initial settings merely repeats the disturbance of other areas of the mirror and the reflected image.
The use of a thin stretched mirror creates diffusion and concentration of image rays that create black holes and bright spots. Even with a thin stretched mirror it was found that when one key point of the reflected image is moved into register with corresponding a key point on the template a previous adjustment made in the location of another key point is disturbed. Consequently, repeated adjustments are required to restore previous settings. Also with a thin stretched mirror, if the controls are connected to the mirror surface, then imperfections, such as waves or blips, in the reflecting surface appear on the reflected image.
While the known device, using a stretched mirror under tension, for producing in-scale aerial photographs has been satisfactorily used for mapping relatively straight line, linear geographic areas, the known device is not satisfactory for mapping relatively large geographic areas due to the problems encountered in controlling deformation of the stretched mirror and with the interaction of deformations made over the surface of the stretched, tensioned mirror. This is particularly mainfest in deforming the tensioned mirror to execute one setting, resulting in a disturbance of a prior executed setting. Therefore, there is need for improved photogrammetric apparatus for making in-scale aerial photographs in which the problems encountered with a stretched, tensioned mirror are overcome. The tension in the stretched mirror must be reduced so that when one setting or deformation of the mirror is made subsequent settings or deformations are not disturbed. This interaction of mirror deformations requires repetitive mirror adjustments to bring and hold the reflected image into registry with the corresponding features on the template. Therefore, an improved photogrammetric device is needed to overcome the problem of interaction of mirror deformations, blips appearing on the reflective surface due to control connection with the mirror, and confining deformation of the reflected surface to selected areas to permit mapping of large geographic areas, as well as, linear areas.